The unique identifier assigned by the sub-group originator
for this sub-group of this P2MP tunnel.
An appropriate value for this object during row creation
when the sub-group origin in
cmplsTeP2mpTunnelDestSubGroupOrigin is the local LSR can
be obtained by reading cmplsTeP2mpTunnelSubGroupIDNext.
At an egress, there is no downstream sub-group ID. This
object should return the value received from upstream and
reported in cmplsTeP2mpTunnelDestSrcSubGroupID.